fix(theme): distinguish active tabs in affected palettes

[oiahoon]

Summary

• add an optional palette-level color for the current MiniTabline entry
• use it for both current and modified-current tabs
• set Abyss, Desert, and Midnight to their red secondary color while preserving the existing fallback for other palettes

Why

The theme-role refactor maps the current tab in these palettes to a khaki color that is close to the hidden tab color, especially when bold styling is disabled. Palette-local values restore a distinct red active state without changing Scorpion, Sol, or unrelated themes.

Fixes #42.

Verification

• targeted StyLua checks for all changed files
• headless Neovim 0.12.4 checks with bold disabled and enabled
• Abyss, Desert, Midnight, Scorpion, and Sol checked in dark and light modes
• all 16 palettes checked in both dark and light modes (32 combinations)
• 100-run startup benchmark
• git diff --check

[uhs-robert]

Thank you for taking the time to put this together. I am a bit busy this weekend but I will hopefully find some time over the next few days to review and test.

I also haven't taken an indepth look at this issue and will need to install Mini agains to check this issue/pull request out. But, just to clarify, this issue only occurs with Midnight and does not impact any other themes?

There are other themes which also apply overrides to primary/secondary, I imagine if the issue impacts Midnight then it would also impact them as well. Can you verify that this issue does not occur with these themes?

• Abyss
• Desert
• Midnight
• Scorpion
• Sol

[oiahoon]

Verified all five palettes in both dark and light modes with bold disabled. Abyss and Desert had the same low-distinction khaki pairing as Midnight, so 7875430 adds the same palette-local red override to those two. Scorpion and Sol already remain distinct and are unchanged.